Filtration of solutions containing gelatinous solids is extremely difficult and slow through normal filter cloths and generally blinding of the cloth occurs with consequent cessation of flow through the filter.
Normal procedure for the improvement in filtration rate involves the use of a filter aid which protects the filter cloth and retains an open structure allowing sufficient flow of filtrate and also clarification of the suspended solids by entrapment in the porous cake.
Diatomaceous earth is used worldwide as a filter aid and consists of the skeletal remains of aquatic plants which are mined in several centers around the world.
Australia has several diatomaceous earth deposits, none of which is entirely suitable as a filter aid because of contamination by clay, sand and iron salts. As a result all filter aid grade diatomaceous earth (DE) used within Australia is imported, mostly from the USA. This involves a large freight cost, particularly because of the low bulk density of the DE, and the necessity to keep large stocks to overcome delivery delays.
Rice hulls are produced as a by-product of the rice milling process in the separation from the rice grain. The hulls are suitable for use as a fuel with an energy level equivalent to that of brown coal. On combustion the hulls generate an ash product which is extremely pure in SiO.sub.2, even more so than DE, low in carbon (approximately 2%) and very porous, indicating it may be suitable as a DE equivalent.
DE is processed by calcining in the presence of fluxing agents and then classified to obtain a range of products with a range of particle size distributions. These products are produced to correspond to various filtration rates with in general the coarser the distribution the higher the flow through the material.
Particle size distributions of the various DE grades have indicated that the particles are generally finer than 50 microns even for the higher filtration rate grades.
Filtration of various solutions, for example beer, wine, glucose syrup etc, results in a relative clarity of filtrate dependent on the grade of DE used. The coarser the grade the poorer the clarity.